The influence of cigarette paper on CO deliveries has been described in a number of studies. Two mechanisms play a major role: dilution and diffusion through the cigarette paper. The dilution is dependant on air permeability. The diffusion is dependant on the real porous structure of the paper. As the measurement of paper permeability does not allow to evaluate diffusion, cigarette papers having the same air permeability can give different CO deliveries. The objective of this study was to have a tool which evaluates the capacity of the cigarette paper to diffuse CO, in order to better explain and control CO deliveries. For this, we have studied different cigarette papers having identical air permeability, but giving different CO deliveries which could only be explained by differences in the porous structure of the paper.The methods we used to evaluate these papers were the following: - Direct measurement of CO diffusion on unlit cigarettes,- Diffusional Conductance Index based on electrical conductance of cigarette papers, - Gas diffusivity measurement through the cigarette paper, - Mercury porosimetry comparisons between the methods, their applicability and limitations will be given. Simple ways of measuring paper porous structure will be proposed.